1. Field of the Invention
The present invention relates to a magnetoresistive element and a magnetic memory using the same.
2. Description of the Related Art
Recently, techniques (spin electronics) using the degree of freedom of an electron spin are being studied. The state density of a ferromagnetic material polarizes near the Fermi surface for each of electrons having up-spin and down-spin. Electrons near the Fermi energy are conducted, and the conductive electrons and spin are hybridized in a ferromagnetic material. An electric current is presumably spin-polarized because the spin has itinerancy. When a ferromagnetic material is regarded as a spin injection source, the possibility of the construction of a high-level function device using the degree of freedom of spin greatly expands.
In addition, nanomaterial design has presently become possible as the deposition techniques and micropatterning techniques advance. When a hybrid structure having a ferromagnetic material is artificially formed in a nanoscale region, the quantum interference effect appears, and many physical phenomena unobservable in any single substance are observed. In particular, a magnetic tunnel junction (MTJ) element as a magnetoresistive element has a basic structure including a first ferromagnetic layer, tunnel barrier layer, and second ferromagnetic layer, and shows the tunneling magnetoresistive (TMR) effect. This MTJ element has been applied to a head for a 100-Mbpsi [bits per square inch] class HDD or a magnetic random access memory (MRAM).
The MRAM stores a binary digit (1 or 0) by using the change in relative angle of the magnetization in the magnetic layer included in the MTJ element, and hence is nonvolatile. Also, since the magnetization switching speed is a few nanoseconds, the MRAM is capable of high-speed data write and read. Accordingly, the MRAM is expected as a next-generation high-speed nonvolatile memory. Furthermore, when using a method called spin transfer magnetization reversal that controls magnetization by using a spin-polarized current, the current density is increased by decreasing the cell size of the MRAM. This makes it possible to readily reverse the magnetization of a magnetic material, and achieve a high-density and low-power-consumption MRAM.
In addition, it is recently theoretically demonstrated that a magnetoresistive ratio of 1,000% is obtained by using MgO as a tunnel barrier layer, and this demonstration is attracting attention (e.g., reference 1: Butler W. H., Zhang X. G., Shulthess T. C., MacLaren J. M., “Spin-dependent tunneling conductance of Fe/MgO/Fe sandwiches”, Phys. Rev. B, Vol. 63, 054416 [2001]). That is, when MgO is crystallized, only electrons having a specific wave number can selectively be conducted by tunneling into the MgO from a ferromagnetic layer while maintaining the wave number. In this state, a giant magnetoresistive effect appears because the spin polarization ratio is large in a specific crystal orientation. Therefore, increasing the magnetoresistive effect of the MTJ element directly leads to a high density and low power consumption of the MRAM.
On the other hand, increasing the integration degree of a magnetoresistive element is essential in increasing the density of a nonvolatile memory. However, a ferromagnetic material forming a magnetoresistive element decreases the heat disturbance resistance as the element size decreases. Accordingly, it is important to improve the magnetic anisotropy and heat disturbance resistance of a ferromagnetic material. In an in-plane magnetization MTJ element, the magnetization of a ferromagnetic material points in the direction of the plane of the element. To increase the integration degree by using this in-plane magnetization MTJ element, attempts have been made to increase the magnetic anisotropy by using the magnetic shape anisotropy. Since the heat disturbance resistance is sensitive to the element shape, however, the processing accuracy is a factor of the characteristic distribution. Also, to achieve the magnetic shape anisotropy, a general approach is to extend the element shape in a specific direction in the plane. Unfortunately, this method is unsuitable to increasing the integration degree because the area occupied by the element increases.
To solve this problem, attempts have recently been made to construct an MRAM using a perpendicular magnetization MTJ element in which the magnetization of a ferromagnetic material points in a direction perpendicular to the film surface. In the perpendicular magnetization MTJ element, a material having large magnetocrystalline anisotropy is generally used as the ferromagnetic material. Since the magnetization of this material points in a specific crystal direction, the magnitude of magnetocrystalline anisotropy can be controlled by changing the composition ratio and crystallinity of the constituent elements. Accordingly, the direction of magnetization can be controlled by changing the crystal growth direction. Also, since the ferromagnetic material itself has high magnetocrystalline anisotropy, the aspect ratio of the element can be set to 1. In addition, the element is suitable to increasing the integration degree because the heat disturbance resistance is high. When these factors are taken into consideration, it is very important to form a perpendicular magnetization MTJ element having a large magnetoresistive effect in order to achieve a high integration degree and low power consumption of the MRAM.